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METHOD FOR MANUFACTURING COKE, COKE, AND METHOD FOR EVALUATING HOMOGENEITY
OF COAL BLEND

Abstract

A method for manufacturing coke having a high strength and excellent
extrusion capability. The method includes a preparing step of blending
two or more coal brands to prepare a coal blend, a stirring and mixing
step of stirring and mixing the coal blend to disintegrate at least a
part of pseudo-particles that have been formed by agglomeration of coal
particles in the coal blend, and a carbonizing step of charging the
stirred and mixed coal blend into a coke oven to carbonize the stirred
and mixed coal blend. Additionally, a mixing apparatus is used in the
stirring and mixing step that has a capability of controlling a degree of
mixing of the coal blend to be 0.85 or more at 60 seconds after start of
a mixing operation. The degree of mixing is calculated by the following
equation (1):
degree of mixing=(.sigma.C.sub.0-.sigma.C)/(.sigma.C.sub.0-.sigma.Cf)
(1).

1. A method for manufacturing coke, the method comprising: a preparing
step of blending two or more coal brands to prepare a coal blend; a
stirring and mixing step of stirring and mixing the coal blend to
disintegrate at least a part of pseudo-particles that have been formed by
agglomeration of coal particles in the coal blend; and a carbonizing step
of charging the stirred and mixed coal blend into a coke oven to
carbonize the stirred and mixed coal blend, wherein: a mixing apparatus
is used in the stirring and mixing step, the mixing apparatus having a
capability of controlling a degree of mixing of the coal blend to be 0.85
or more at 60 seconds after start of a mixing operation, the degree of
mixing being calculated by the following equation (1): degree of
mixing=(.sigma.C.sub.0-.sigma.C)/(.sigma.C.sub.0-.sigma.Cf) (1) where
the degree of mixing is a value calculated from the standard deviations
of characteristic values which are respectively determined for samples
taken from the stirred and mixed coal blend, .sigma.C.sub.0 denotes the
standard deviation of characteristic values when mixing is not performed
at all, .sigma.Cf denotes the standard deviation of characteristic values
when mixing has been completely performed, and .sigma.C denotes the
standard deviation of characteristic values of the samples taken.

2. The method according to claim 1, wherein the coal blend is stirred and
mixed in the stirring and mixing step so that the degree of mixing is
0.85 or more.

3. The method according to claim 1, wherein the coal blend has a value of
(.sigma.C.sub.0-.sigma.Cf)/Cave of 0.40 or more, where Cave denotes an
average value of the determined characteristic values.

4. The method according to claim 1, wherein the degree of mixing is a
value calculated from the standard deviations of characteristic values
which are respectively determined for the samples having a weight of 2 g
or less, the samples being taken from plural positions of the stirred and
mixed coal blend.

5. The method according to claim 1, wherein the preparing step includes a
step of pulverizing the two or more coal brands before blending the two
or more coal brands.

6. The method according to claim 1, wherein the preparing step includes a
step of controlling the moisture contents of the two or more coal brands.

7. The method according to claim 1, comprising performing the stirring
and mixing step to a coal blend having a moisture content of 6 mass % or
more.

8. A method for evaluating homogeneity of a coal blend when coke is
manufactured by the method according to claim 1, the method comprising
the steps of: taking samples from any positions of a coal blend before
and after the stirring and mixing step; determining a characteristic
value of each of the samples; calculating a degree of mixing from the
standard deviations of characteristic values which are respectively
determined for the samples having a value of
(.sigma.C.sub.0-.sigma.Cf)/Cave of 0.40 or more; and evaluating the
homogeneity of the coal blend on the basis of the degree of mixing which
is calculated by the following equation (3): degree of
mixing=(.sigma.C.sub.0-.sigma.C)/(.sigma.C.sub.0-.sigma.Cf) (3) where
.sigma.C.sub.0 denotes the standard deviation of characteristic values
when mixing is not performed at all, .sigma.Cf denotes the standard
deviation of characteristic values when mixing has been completely
performed, .sigma.C denotes the standard deviation of characteristic
values in an any state of mixing, and Cave denotes the average value of
the determined characteristic values.

9. The method according to claim 8, wherein the characteristic values are
respectively the sulfur concentrations of the samples.

10. The method according to claim 8, wherein the characteristic values
are respectively determined for the samples having a weight of 2 or less,
the samples being taken from plural positions of the coal blend before
and after the stirring and mixing step.

11. The method according to claim 9, wherein the sulfur concentration is
determined by a carbon-sulfur analyzer.

12. The method according to claim 8, wherein a sensitizer is mixed into
the coal blend and then the coal blend is stirred.

13. The method according to claim 12, wherein the sensitizer is at least
one of oil coke, coal-tar itch and as halt pitch.

14. A method for manufacturing coke, the method comprising: a preparing
step of blending two or more coal brands to prepare a coal, blend; a
stirring and mixing step of stirring and mixing the coal blend to
disintegrate at least a part of pseudo-particles that have been formed by
agglomeration of coal particles in the coal blend; and a carbonizing step
of charging the stirred and mixed coal blend into a coke oven to
carbonize the stirred and mixed coal blend, wherein: a mixing apparatus
is used in the stirring and mixing step, the mixing apparatus having a
capability of controlling a degree of mixing of the coal blend to be 0.85
or more at 60 seconds after start of a mixing operation, the degree of
mixing being calculated by the following equation (2): degree of
mixing=(.sigma.TS.sub.0-.sigma.TS)/(.sigma.TS.sub.0-.sigma.TSf) (2)
where the degree of mixing is a value calculated from the standard
deviations of sulfur concentrations which are respectively determined for
the samples taken from the stirred and mixed coal blend, .sigma.TS.sub.0
denotes the standard deviation of sulfur concentrations when mixing is
not performed at all, .sigma.TSf denotes the standard deviation of sulfur
concentrations when mixing has been completely performed, and .sigma.TS
denotes the standard deviation of sulfur concentrations of the samples
taken.

15. The method according to claim 14, wherein the coal blend is stirred
and mixed in the stirring and mixing step so that the degree of mixing is
0.85 or more.

16. The method according to claim 14, wherein the coal blend has a value
of (.sigma.TS.sub.0-.sigma.TSf)/TSave of 0.40 or more, where TSave
denotes an average value of the determined sulfur concentrations.

17. The method according to claim 14, wherein the degree of mixing is a
value calculated from the standard deviations of sulfur concentration
which are respectively determined for the samples having a weight of 2 g
or less, the samples being taken from plural positions of the stirred and
mixed coal blend.

18. The method according to claim 14, wherein the preparing step includes
a step of pulverizing the two or more coal brands before blending the two
or more coal brands.

19. The method according to claim 14, wherein the preparing step includes
a step of controlling the moisture contents of the two or more coal
brands.

20. The method according to claim 14, comprising performing the stirring
and mixing step to a coal blend having a moisture content of 6 mass % or
more.

Description

TECHNICAL FIELD

[0001] The present disclosure relates to a method for manufacturing coke
by charging a coal blend into a coke oven and carbonizing the coal blend,
coke manufactured by using the method, and a method for evaluating the
homogeneity of a coal blend.

BACKGROUND ART

[0002] In general, in a coke oven, various kinds of operational problems
occur due to the progress of aging. Among such operational problems,
"sticker" is a very serious operational problem that it is not possible
to discharge manufactured coke from a coke oven. Since "sticker"
occurrence forces change in a schedule for manufacturing coke, the amount
of produced coke becomes decreasing and the life of the coke oven becomes
shortened due to induced damage to the oven body. Therefore, decreasing
the frequency of occurrence of "sticker" is given first priority in the
operation.

[0003] A mechanism by which "sticker" occurs is roughly described as
follows. The operation of a general chamber-type coke oven involves
carbonizing a coal blend which has been charged into a carbonizing
chamber to form a coke cake sequentially from the oven wall side due to
heat transferred from a combustion chamber adjacent to the carbonizing
chamber. Here, usually, since a coke cake shrinks due to carbonization, a
gap (hereinafter, referred to as "clearance") is formed between the oven
wall and the outer surface of the coke cake. Formation of the clearance
facilitates to discharge (extrude) the coke cake from the coke oven.

[0004] However, since an insufficient amount of shrinkage of a coke cake
does not form a sufficiently large clearance, "sticker" occurs due to
increased frictional resistance between the oven walls and the outer
surface of the coke cake when the coke cake is extruded. Also, in the
case where irregularity of the oven wall surface is large, "sticker"
occurs due to increased frictional resistance between the oven walls and
the outer surface of the coke cake. The irregularity of the oven wall
surface is increased as a result of the abrasion and removal of oven wall
bricks, an increase in the amount of carbon adhered to the oven walls,
and so forth due to the aging of the coke oven. Therefore, frequency of
occurrence of "sticker" inevitably increases due to the aging of a coke
oven. In consideration of such a background, in case of operating of an
aging coke oven, various countermeasures are implemented in order to
decrease the frequency of occurrence of "sticker".

[0005] A moisture-coal operation can be mentioned for an example of
countermeasures aimed at decreasing the frequency of occurrence of
"sticker". The moisture-coal operation involving using a coal blend of
which the moisture content is not actively decreased from the content
(about 8 mass % to 14 mass %, although it depends on season and weather)
when the coal blend is piled in a coal yard. The moisture-coal operation
is widely used as the simplest and effective method. Increasing the
moisture content of a coal blend makes the bulk density of a charged coal
blend decrease and there is an increase in clearance or the like, thereby
reducing frictional resistance between the oven walls and the surface of
a coke cake when the coke cake is extruded. At the result thereof, it is
possible to decrease the frequency of occurrence of "sticker".

[0006] As a specific example of the method described above, Patent
Literature 1 discloses a technique involving carbonizing a coal blend in
a coke oven after the moisture content of the coal blend has been
controlled by a coal-moisture-controlling apparatus. The technique
involves determining the target moisture content of a coal blend
necessary to achieve desired clearance on the basis of the relationship
derived in advance between the moisture content of the coal blend and
clearance, and controlling the heat input to a coal-moisture-controlling
apparatus so that the total moisture content of the coal blend at the
exit of the coal-moisture-controlling apparatus is the target moisture
content. Such controlling decreases the frequency of occurrence of
"sticker".

[0007] In addition, Patent Literature 2 discloses a technique involving
adding water locally to a coal in a coal tower and charging the coal into
a carbonizing chamber via a larry car. The technique makes clearance
increase due to an increase in the shrinkage ratio of coke in a part of
the coal having an increased moisture content existing locally in the
carbonizing chamber. The increase of clearance results in a decrease in
the frequency of occurrence of "sticker".

[0008] The conventional techniques described above have a common technical
feature. The feature is increasing the moisture content of coal to be
charged into a coke oven to form a clearance with an increased shrinkage
ratio when carbonization is performed.

[0009] On the other hand, a blast furnace operation recently involves
blowing pulverized coal into a blast furnace in order to decrease the
amount of coke used. The operation needs coke having relatively higher
strength, in particular, coke excellent in terms of drum strength which
is determined by using a drum strength test method prescribed in JIS K
2151 is necessary. The blast furnace requires sufficient gas permeability
and liquid permeability so as to progress the reducing reaction of iron
ore efficiently and stably. In case of insufficient coke strength, there
occurs a problem of a decrease in gas permeability and liquid
permeability in a hollow space called a "raceway" which is formed in
front of a tuyere and the lower part of the blast furnace due to the
collision of coke particles.

[0010] Techniques for improving coke strength are largely classified into
three groups in terms of processes in which they are used, that is,
pretreatment techniques, blending techniques, and carbonizing techniques.
In particular, pretreatment techniques are considered to be important,
because the techniques makes it possible to design equipment so that
there is no limitation on the productivity of a coke oven without an
increase in the costs for coal blend. Such pretreatment techniques are
classified mainly into the following two groups in terms of the approach
to coke strength.

[0011] (1) A technique for improving the charged bulk density of a coal
blend (hereinafter, referred to as "technique (1)") [0012] (2) A
technique for homogenizing a coal blend (hereinafter, referred to as
"technique (2)")

[0013] The technique (1) is intended to decrease the number of pore
defects which influence coke strength. The technique (1) involves
mechanically compacting a coal blend to improve charged bulk density and
charging the coal blend into a coke oven so as to reduce the
interparticle space of the coal. The technique (1) results in an
improvement in coke strength. Specific examples of the technique include
a method of charging coal briquettes partially, a stamping method, and a
method of decreasing the moisture content of a coal blend in order to
decrease the interparticle adhesiveness of the coal to improve the
charged bulk density (refer to Non Patent Literature 1). However, a
process in which the moisture content of a coal blend is decreased by
using a moisture-controlling apparatus or a preheating apparatus is
introduced into an operation of many coke ovens.

[0014] In contrast, the technique (2) is intended to increase the strength
of a portion of coke having the lowest strength. Since coal is
fundamentally composed of textures having different properties in terms
of various thermal and mechanical properties, coal is very inhomogeneous.

Naturally, the structure of coke, which is manufactured from such
inhomogeneous coal, is also inhomogeneous. Generally, the strength of a
brittle material such as coke is described on the basis of a weakest link
model and determined by the strength of a portion having the lowest
strength in the material. Therefore, since the strength of coke is
homogenized by homogenizing the structure of the coke, the technique (2)
makes it possible to improve the strength of the entire coke.

[0015] Examples of a method for the technique (2) include a method in
which the particle size of coal is controlled (refer to Non Patent
Literature 1). The method of controlling the particle size of coal is
basically intended to homogenize the structure of coke by finely
pulverizing coal. Also, a method is known which is intended to homogenize
the structure of coke by treating coal with a coal-mixing machine such as
a drum mixer in order to increase the degree of mixing of the coal (refer
to Non Patent Literature 2). However, it has been clarified by
conventional research that, without being treated with a coal-mixing
machine, a coal blend which is used in a coke-making process is
sufficiently mixed, for example, at connection parts of a belt conveyer
in a transportation process (refer to Non Patent Literature 2).
Therefore, in many coke plants, consideration is given to homogenize the
structure of coke without using a coal-mixing machine nowadays.

[0021] In order to stably operate a coke oven and a blast furnace, it is
necessary to realize both the achievement of a sufficient clearance due
to the shrinkage of a coal blend and the achievement of sufficient coke
strength at the same time.

[0022] However, since the techniques according to Patent Literature 1 and
Patent Literature 2 and techniques (1) and (2) have the following
problems, it is a fact that both are not realized at the same time
currently.

[0023] The technique according to Patent Literature 1 involves controlling
the moisture content of a coal blend to control clearance in order to
achieve a target clearance which is necessary to inhibit "sticker" from
occurring. Therefore, although the technique is effective for inhibiting
"sticker" from occurring, it is not possible to inhibit a decrease in
coke strength. Also, since the technique according to Patent Literature 2
involves controlling the moisture content of a coal blend to control
clearance, it is not possible to inhibit a decrease in coke strength.

[0024] In contrast, although the technique (1) is effective for improving
coke strength, since there is a decrease in clearance due to an increase
in the bulk density of a coal blend, it is not possible to inhibit
"sticker" from occurring.

[0025] The technique (2) is effective not only for improving coke strength
but also for achieving a sufficient clearance (refer to Non Patent
Literature 3). However, in the case where the moisture content of a coal
blend is high, since coal particles agglomerate through water even if a
coal blend is pulverized into a small particle size, large
pseudo-particles are formed. The pseudo-particles remain indisintegrated
even if the pseudo-particles in the coal blend are subjected to stirring
and mixing using a coal-mixing machine such as a drum mixer which mainly
involves convective mixing, and therefore it is not possible to achieve
sufficient coke strength due to inhomogeneous structure formed inside the
coke. In addition, the influence of the behavior and configuration such
as size and structure of the pseudo-particles on coke strength has not
been sufficiently clarified. Therefore, a preferable method for breaking
the pseudo-particles has not been clarified yet.

[0026] It is necessary to decrease the moisture content of a coal blend in
order to improve coke strength for the reason described above. However,
since there is an increase in the frequency of occurrence of "sticker" in
the case of low moisture content, there is a trend toward rather
increasing the moisture content of a coal blend. It is a fact that
currently an aging coke oven which has been used for more than 40 years
is operated with the moisture content of a coal blend being maintained at
a high level at the sacrifice of coke strength.

[0027] On the other hand, in the case where pseudo-particles are
disintegrated in order to improve homogeneity, it is not clear what kind
of index should be used to evaluate homogeneity or what level of
homogeneity should be provided in order to obtain coke having a desired
strength.

[0028] The present disclosure has been completed in view of the problems
described above, and an object of the present disclosure is to provide
coke having a high strength and excellent discharging property from a
coke oven and a method for manufacturing the coke, and, in addition, to
provide a method for quantitatively evaluating the homogeneity of a coal
blend.

Solution to Problem

[0029] The present inventors, in view of the problems described above,
diligently conducted investigations regarding the influence of the
homogeneity of a coal blend on coke strength from the viewpoint of
pseudo-particles.

[0030] As a result, the present inventors found that it is highly probable
that the homogeneity on the order of millimeters of a coal blend
influences coke strength. The present inventors found that there is a
decrease in homogeneity in the case where the number of particles in coal
of a single brand (hereinafter, refers to as "single coal brand") having
a particle diameter of several millimeters in a coal blend is large and
that, even in the case where the particle diameter of a single coal brand
is small, in the case where the coal blend is not sufficiently mixed and
where moisture content is more than 6 [mass %], there is an increase in
the mass fraction of pseudo-particles having a particle diameter of 1
[mm] or more, which results in a decrease in homogeneity on the order of
millimeters.

[0031] Also, the present inventors found that determining some properties
of a coal blend which satisfy specific conditions is effective as a
method for quantitatively evaluating the homogeneity on the order of
millimeters of a coal blend. For example, it is possible to
quantitatively evaluate homogeneity by determining a change in sulfur
concentration in a coal blend.

[0032] As described above, the present inventors clarified what kind of
criterion should be used in order to evaluate the homogeneity on the
order of millimeters of a coal blend. In addition, the present inventors
reached a conclusion that, by stirring and mixing a coal blend by using a
mixer having a capability for satisfying such a criterion, it is possible
to prevent a decrease in coke strength even in the case where the
moisture content of the coal blend is more than 6 [mass %].

[0033] The present disclosure has been completed on the basis of the
knowledge described above. Exemplary disclosed embodiments include as
follows.

[1] A method for manufacturing coke including: a preparing step of
blending two or more coal brands to prepare a coal blend; a stirring and
mixing step of stirring and mixing the coal blend which has been prepared
in the preparing step to disintegrate at least a part of pseudo-particles
that have been formed by agglomeration of coal particles in the coal
blend; and a carbonizing step of charging the stirred and mixed coal
blend into a coke oven to carbonize the stirred and mixed coal blend,
wherein a mixing apparatus is used in the stirring and mixing step, the
mixing apparatus having a capability of controlling degree of mixing of
the coal blend to be 0.85 or more at 60 seconds after start of a mixing
operation, the degree of mixing being calculated by a following equation
(1):

where the degree of mixing is a value calculated from the standard
deviations of characteristic values which are respectively determined for
samples taken from the stirred and mixed coal blend, .sigma.C.sub.0
denotes the standard deviation of characteristic values when mixing is
not performed at all, .sigma.Cf denotes the standard deviation of
characteristic values when mixing has been completely performed, .sigma.C
denotes the standard deviation of characteristic values of the samples
taken. [2] A method for manufacturing coke including: a preparing step of
blending two or more coal brands to prepare a coal blend; a stirring and
mixing step of stirring and mixing the coal blend which has been prepared
in the preparing step to disintegrate at least a part of pseudo-particles
that have been formed by agglomeration of coal particles in the coal
blend; and a carbonizing step of charging the stirred and mixed coal
blend into a coke oven to carbonize the stirred and mixed coal blend,
wherein a mixing apparatus is used in the stirring and mixing step, the
mixing apparatus having a capability of controlling degree of mixing of
the coal blend to be 0.85 or more at 60 seconds after start of a mixing
operation, the degree of mixing being calculated by a following equation
(2):

where the degree of mixing is a value calculated from the standard
deviations of sulfur concentrations which are respectively determined for
the samples taken from the stirred and mixed coal blend, .sigma.TS.sub.0
denotes the standard deviation of sulfur concentrations when mixing is
not performed at all, .sigma.TSf denotes the standard deviation of sulfur
concentrations when mixing has been completely performed, .sigma.TS
denotes the standard deviation of sulfur concentrations of the samples
taken. [3] The method according to item [1] or [2] above, wherein the
coal blend is stirred and mixed in the stirring and mixing step so that
the degree of mixing is 0.85 or more. [4] The method according to item
[1] or [3] above, wherein the coal blend has a value of
(.sigma.C.sub.0-.sigma.Cf)/Cave of 0.40 or more, where Cave denotes an
average value of the determined characteristic values. [5] The method
according to item [2] or [3] above, wherein the coal blend has a value of
(.sigma.TS.sub.0-.sigma.TSf)/TSave of 0.40 or more, where TSave denotes
an average value of the determined sulfur concentrations. [6] The method
according to any one of items [1], [3], and [4] above, wherein the degree
of mixing is a value calculated from the standard deviations of
characteristic values which are respectively determined for the samples
having a weight of 2 g or less, the sample being taken from plural
positions of the stirred and mixed coal blend. [7] The method according
to any one of items [2], [3], and [5] above, wherein the degree of mixing
is a value calculated from the standard deviations of sulfur
concentration which are respectively determined for the samples having a
weight of 2 g or less, the sample being taken from plural positions of
the stirred and mixed coal blend. [8] The method according to any one of
items [1] to [7] above, wherein the preparing step includes a step of
pulverizing two or more coal brands before blending the two or more coal
brands. [9] The method according to any one of items [1] to [8] above,
wherein the preparing step includes a step of controlling the moisture
contents of the two or more coal brands. [10] The method according to any
one of items [1] to [9] above, including performing the stirring and
mixing step to a coal blend having a moisture content of 6 mass % or
more. [11] Coke manufactured by the method according to any one of items
[1] to [10] above. [12] A method for evaluating homogeneity of a coal
blend when coke is manufactured by the method according to any one of
items [1], [3], [4], [6], and [8] to [10] above, the method including
steps of: taking samples from any positions of a coal blend before and
after a stirring and mixing step; determining the characteristic value of
each of the samples; calculating a degree of mixing from the standard
deviations of characteristic values which are respectively determined for
the samples having a value of (.sigma.C.sub.0-.sigma.Cf)/Cave of 0.40 or
more; and evaluating the homogeneity of the coal blend on the basis of
the degree of mixing which is calculated by equation (3):

where .sigma.C.sub.0 denotes the standard deviation of characteristic
values when mixing is not performed at all, .sigma.Cf denotes the
standard deviation of characteristic values when mixing has been
completely performed, .sigma.C denotes the standard deviation of
characteristic values in an any state of mixing, and Cave denotes the
average value of the determined characteristic values. [13] The method
according to item [12] above, wherein the characteristic values are
respectively the sulfur concentrations the samples. [14] The method
according to item [12] or [13] above, wherein the characteristic values
are respectively determined for the samples having a weight of 2 g or
less, the sample being taken from plural positions of the coal blend
before and after the stirring and mixing step. [15] The method according
to item [13] or [14] above, wherein the sulfur concentration is
determined by using a carbon-sulfur analyzer. [16] The method according
to any one of items [12] to [15] above, wherein a sensitizer is mixed
into the coal blend and then the coal blend stirred. [17] The method
according to item [16] above, wherein the sensitizer is at least one of
oil coke, coal-tar pitch, and asphalt pitch.

Advantageous Effects

[0034] According to the present disclosure, it is possible to obtain coke
having a high strength and excellent extrusion capability from a coke
oven. In addition, it is possible to evaluate the homogeneity of a coal
blend.

BRIEF DESCRIPTION OF DRAWINGS

[0035] FIG. 1 is a diagram illustrating the relationship between the
moisture content of a coal blend and particle size distribution.

[0036] FIG. 2 is a schematic diagram illustrating a method for evaluating
clearance.

[0037] FIG. 3 is a diagram illustrating the relationship between the
moisture content of a single coal brand before a mixing step and coke
strength.

[0038] FIG. 4 is a diagram illustrating the relationship between the mass
fraction of particles having a particle diameter of 1 [mm] or more in
coal and coke strength.

[0039] FIG. 5 is a diagram illustrating the relationship between the
moisture content of a single coal brand before a mixing step and the
evaluation results of an optical texture of coke.

[0040] FIG. 6 is a diagram illustrating the relationship between the
stirring and mixing time of a mixer and the degree of mixing.

[0041] FIG. 7 is a diagram illustrating the relationship between the
degree of mixing after 60 seconds and the disintegrated level.

[0042] FIG. 8 is a diagram illustrating the relationship between the
blending ratio of delayed oil coke and (.sigma.C.sub.0-.sigma.Cf)/Cave.

[0043] FIG. 9 is a diagram illustrating the relationship between the
degree of mixing of a coal blend and coke strength.

[0044] FIG. 10 is a diagram illustrating the relationship between a
treating time (average retention time) of a mixer and the degree of
mixing of a coal blend.

[0045] FIG. 11 is a diagram illustrating the relationship between the coke
strength derived from a coal blend before a mixer treatment and the coke
strength derived from a coal blend after 60 seconds of a mixer treatment
(treatment for an average retention time of 60 seconds).

DESCRIPTION OF EMBODIMENTS

[0046] Hereafter, the process of investigations through which the present
disclosure was conceived will be described in detail, and then an
exemplary embodiment of the present disclosure will be described.

[0047] [Relationship Between the Homogeneity of a Coal Blend and Coke
Strength]

[0048] First, the present inventors conducted investigations regarding the
relationship between the moisture content of a coal blend and the state
of pseudo-particles formed (Experiment 1), and then conducted
investigations regarding the influence of the formation of
pseudo-particles on the homogeneity of a coal blend and coke strength
(Experiments 2 and 3).

Experiment 1

[0049] A coal blend having general properties was used as an experimental
sample. The general properties are ones for manufacturing metallurgical
coke. Table 1 represents the properties (mean maximum reflectance Ro [%],
Gieseler maximum fluidity log MF [log ddpm], volatile matter content VM
[mass %], and ash matter content Ash [mass %]) and blending ratios [mass
%] of four kinds of single coal brands (coal A through coal D) of which
the coal blend was composed. Table 2 represents a weighted average
according to the blending ratios regarding each of properties of the coal
blend. Mean maximum reflectance was determined in accordance with JIS M
8816. Gieseler maximum fluidity was determined in accordance with JIS M
8801. Volatile and ash matter contents were determined in accordance with
JIS M 8812 and the contents were on a dry basis.

[0050] The coal blend was pulverized and prepared so as to have a particle
size distribution simulating a practical operation (3 [mm] or less: 75
[%], more than 3 [mm] and 6 [mm] or less: 15 [%], and more than 6 [mm]:
10 [%], in terms of mass % on a dry basis). A method for preparing coal
blends having homogeneous moisture content involves: heating the coal
blend to a temperature of 107 [.degree. C.] so as to have a moisture
content of 0 [mass %]; thereafter adding water; and leaving the samples
untouched for 24 hours. The coal blends were prepared by the method, the
coal blends having eight kinds of moisture contents (0, 4, 6, 7, 8, 9,
10, and 12 in terms of [mass %]). Subsequently, the coal blends were
sieved by using a sieve-shaking machine with a constant impact being
repeatedly applied for five minutes, and then particle size distribution
was determined. In an ordinary determination of the particle size
distribution of coal, sieve analysis is performed after a coal sample is
dried so that pseudo-particles are broken. In contrast, in the present
experiment, performing sieve analysis with the moisture content of coal
being maintained makes it possible to determine the particle size
distribution of pseudo-particles.

[0051] Table 3 represents the determination results of particle size
distribution for each of the moisture contents of a coal blend. FIG. 1
illustrates the relationship between the moisture content of a coal blend
and particle size distribution.

[0052] As Table 3 and FIG. 1 indicate, in the case where the moisture
content of a coal blend was 4 [mass %] or less, the particle size
distribution was not substantially different from the initial particle
size distribution (in the case of a moisture content of 0 [mass %]). In
contrast, in the case where the moisture content of a coal blend was more
than around 6 [mass %], there was a marked increase in the mass
proportion of particles in particular having a large particle diameter of
1 [mm] or more. From the results of the observation of the particles
having a large particle diameter of 1 [mm] or more by using an optical
microscope, since there were a large number of pseudo-particles, it was
confirmed that the formation of pseudo-particles progressed in the case
where the moisture content of a coal blend was more than around 6 [mass
%] and that such pseudo-particles were not broken even by the impact
applied in the sieve-shaking process.

[0053] In order to investigate the influence of the formation of
pseudo-particles on the homogeneity of a coal blend and coke strength, by
controlling the moisture content of each of coal A through coal D which
were used in Experiment 1 to be 3, 4, 6, 8, and 10 [mass %] in advance,
pseudo-particles were formed. These were charged into a drum mixer which
mainly involves convective mixing and subjected to mixing for 60 seconds
in order to obtain coal blends having the blending ratios given in Table
1. By performing a visual test, it was clarified that there was almost no
change in the particle size distribution of pseudo-particles between
before and after the mixing process. Subsequently, in order to compensate
for the shortage, water was added to the coal blends by performing
spraying so that the moisture content of the coal blends was 10 [mass %],
and the coal blends were then left untouched for 24 hours in order to
obtain homogeneous moisture content.

[0054] The strength of coke obtained from the coal blends as described
above was evaluated through the following procedures.

[0055] By filling 17.1 [kg] of each of the coal blends to a carbonizing
vessel so that the bulk density (based on dry weight) was 725
[kg/m.sup.3], by carbonizing the coal blend with a weight of 10 [kg]
being placed on the top of the carbonizing vessel in an electric furnace
having a furnace wall temperature of 1050 [.degree. C.] for 6 hours, by
removing the carbonizing vessel from the furnace, and then by cooling the
carbonizing vessel with nitrogen gas, coke was obtained. The strength of
the obtained coke was evaluated in accordance with the drum strength test
method prescribed in JIS K 2151. By determining the mass fraction of coke
having a particle diameter of 15 [mm] or more after the coke had been
rotated 150 times at a rotating speed of 15 [rpm], and by calculating the
ratio of the mass fraction to that before the rotation, the ratio
multiplied by 100 was defined as drum index DI (150/15) [-].

[0056] Subsequently, clearance was evaluated through the following
procedures.

[0057] FIGS. 2(a) and (b) are schematic diagrams illustrating a small
simulation retort 1 for evaluating clearance. This small simulation
retort 1 had a length L of 114 [mm], a width W of 190 [mm], a height H of
120 [mm], a bottom panel 11 composed of bricks, a pair of side panels 12a
and 12b composed of metal which stood up from the bottom panel 11, and a
top panel 13 composed of bricks which was placed on the top of the pair
of side panels 12a and 12b. By filling the small simulation retort with
2.244 [kg] of coal blend 2 so that the bulk density (based on dry weight)
was 775 [kg/m.sup.3], by carbonizing the coal blend in an electric
furnace having a furnace wall temperature of 1050 [.degree. C.] for 4
hours and 20 minutes, by removing the retort from the furnace, and then
by cooling the retort with nitrogen gas, a coke cake was obtained. A gap
D between the side surface of the obtained coke cake 3 and each of the
side panels 12a and 12b was determined by using a laser distance meter.
By calculating the average value of the gap D on each of the sides, the
sum of the gaps on both sides was defined as a clearance.

[0058] Moisture contents before a mixing step and the determined results
of coke strength and clearance are given in Table 4. In addition, FIG. 3
illustrates the relationship between the moisture content before a mixing
step and the coke strength.

[0059] As Table 4 and FIG. 3 indicate that there was a sharp decrease in
coke strength in the case where the moisture content before a mixing step
was more than 6 [mass %], although the coke strength was 83.0 or more and
there was almost no change in coke strength in the case where the
moisture content before a mixing step was 6 [mass %] or less. In
contrast, the clearance was almost constant independently of the moisture
content before a mixing step.

[0060] The inventors of the present disclosure consider that there was a
sharp decrease in coke strength in the case where the moisture content
before a mixing step was more than 6 [mass %] for the following reason.
As FIG. 1 illustrates, in the case where the moisture content of a coal
blend is more than 6 [mass %], there is an increase in the mass fraction
of pseudo-particles having a particle diameter of 1 [mm] or more. Also,
in the case of a single coal brand, the formation of pseudo-particles
progresses in the case where the moisture content is more than 6 [mass
%]. It is presumed that, in the case where these pseudo-particles remain
indisintegrated in a stirring and mixing step, a significantly
inhomogeneous coal blend is formed. It is considered that in the case of
coke which is obtained by carbonizing such a coal blend, since a
difference in, for example, thermoplasticity among pseudo-particles
causes large defects, fracturing tends to occur due to the defects.
Accordingly, it is difficult to achieve a high strength.

[0061] In order to verify the presumption described above, the mass
fractions of pseudo-particles having a particle diameter of 1 [mm] or
more in the coal blends used in FIG. 3 to determine coke strength were
determined. FIG. 4 illustrates the results. In addition, the structure of
the obtained coke was observed by using an optical microscope. The
characteristic of the coke structure was classified into four categories,
that is, "leaflet", "fibrous", "mosaic", and "isotropic", and the
proportions of the respective categorized structures were determined.
FIG. 5 illustrates the results.

[0062] As FIG. 4 indicates, it was clarified that, in the case of a coal
blend having a low coke strength of 82.5 or 82.0, the mass fraction of
pseudo-particles having a particle diameter of 1 [mm] or more was higher
than in the case of coal blends having a coke strength of 83.0 or more.
In addition, as FIG. 5 indicates, in the case where the moisture content
before a mixing step is more than 6 [mass %], there was a decrease in the
proportion of a homogeneous mosaic structure, and there was an increase
in the proportions of an isotropic structure and an acicular structure.

[0063] As described above, in the case where the moisture content before a
mixing step is more than 6 [mass %], since the inhomogeneous structure of
coke including a large number of defects therein is formed due to the
formation of pseudo-particles, there is a decrease in strength.
Therefore, the present inventors considered that it is possible to
increase coke strength if pseudo-particles in a coal blend are
disintegrated. Here, the reason why the clearance was almost constant
independently of the moisture content before a mixing step in Table 4 is
considered to be because the moisture content was controlled to be 10%
after a blending step.

[0064] [Degree of Mixing of a Coal Blend]

[0065] On the basis of the results described above, the present inventors
conducted additional investigations. As a result, the present inventors
focus on the degree of stirring and mixing of a coal blend to find that
it is possible to express the disintegrated level of pseudo-particles in
a coal blend in terms of the degree of mixing of the coal blend. The
present inventors find that the degree of mixing can be quantitatively
expressed and used as an index by using the characteristic value of a
coal blend for a method for evaluating the degree of mixing of a coal
blend and by determining a variation in the characteristic value among
samples before and after a stirring and mixing step.

[0066] As an example of a method for evaluating the degree of mixing of a
coal blend, Non Patent Literature 2 discloses the results of the
evaluation of the degree of mixing in the pretreatment process of a coal
blend involving using a drum mixer. The degree of mixing is a general
index for quantifying the homogeneity of powder and is defined by some
equations, and, in any of the definition equations, by using a desired
amount of powder under consideration as a population, by taking plural
samples from the population, and by determining a characteristic value of
each of the samples such as concentration, density, or moisture content,
the degree of mixing is calculated on the basis of a variation (such as
variance, standard deviation, or coefficient of variation) in the
characteristic value among the samples. In Non Patent Literature 2, by
adding a radio isotope as a tracer, the degree of mixing defined by
equation (4) below is evaluated.

[0067] Here, M' denotes the degree of mixing (%), CV.sub.0 denotes the
coefficient of variation (=standard deviation/average value) of tracer
concentration in a complete state of mixing, CV.sub.1 denotes the
coefficient of variation of tracer concentration in a reference state of
mixing, and CV denotes the coefficient of variation of tracer
concentration in some state of mixing.

[0068] As this definition equation indicates, the degree of mixing M' is
an index which approaches 100% as a state of mixing approaches a complete
state of mixing, that is, as homogeneity increases and which, conversely,
approaches 0% as a state of mixing approaches a reference state. In Non
Patent Literature 2, by taking a sample in an amount (about 300 g)
corresponding to the increment shovel in accordance with JIS M 8811-30 as
a single sample and by determining the tracer concentration of the
sample, the degree of mixing is evaluated on the basis of equation (4).
The bulk density of coal varies depending on moisture content and
particle size, and, in the case of general coke making, the bulk density
is about 0.65 g/cm.sup.3 to 0.85 g/cm.sup.3. That is, in the case where
the mass of a sample is about 300 g, the volume of the sample is about
350 cm.sup.3 to 450 cm.sup.3, that is, corresponds to a cube having a
side length of about 7 cm to 8 cm. That is, it can be said that the
degree of mixing according to Non Patent Literature 2 is an index for
evaluating the homogeneity of a comparatively large order, that is, of
about 7 cm to 8 cm.

[0069] However, in the case of the method according to Non Patent
Literature 2, it is not possible to evaluate a change in the degree of
mixing corresponding to the disintegrated level of pseudo-particles
having a particle size illustrated in FIG. 1. Neither information
regarding the relationship between the degree of mixing and coke strength
nor information regarding homogeneity on the order of millimeters which
is considered to have a correlation with coke strength is disclosed.

[0070] Therefore, the present inventors conducted investigations regarding
various characteristic values of coal and the conditions to be satisfied
by the characteristic values in order to evaluate homogeneity on the
order of millimeters. As a result thereof, the present inventors found
that it is preferable to express the degree of mixing by equation (1)
below.

[0071] Here, the degree of mixing is a value calculated from the standard
deviation of characteristic values which are respectively determined for
the samples taken from any positions of a coal blend before and after a
stirring and mixing step. .sigma.C.sub.0 denotes the standard deviation
of characteristic values when mixing is not entirely performed. .sigma.Cf
denotes the standard deviation of characteristic values when mixing has
been completely performed. .sigma.C denotes the standard deviation of
characteristic values in a state of mixing.

[0072] Here, the term "the standard deviation of characteristic values in
a state of mixing" refers to the standard deviation of characteristic
values which are respectively determined for the samples taken in a
certain state of mixing. Although, for example, the constituent chemical
elements in coal or the physical or chemical property may be used as the
characteristic value, it is preferable to use a property whose value
varies depending on coal brand. For example, the content of a particular
chemical element in coal, the content of ash matter, the content of metal
in ash matter, reflectance, the content of a constituent structure, or
thermoplasticity may be used.

[0073] Hereafter, an example in which the sulfur concentration in coal is
used as the characteristic value will be described.

[0074] The degree of mixing can be quantitatively expressed and used as an
index by using sulfur contained in a coal blend for a method for
evaluating the degree of mixing of a coal blend and by determining a
variation in sulfur concentration among samples.

[0075] First, by using sulfur concentration in coal as a characteristic
value, the degree of mixing was calculated. Subsequently, in order to
determine a preferable range of the degree of stirring and mixing in
terms of the degree of mixing, the proportion of disintegrating of
pseudo-particles having a particle diameter of 1 [mm] or more was
calculated and defined as the disintegrated level, and the relationship
between the disintegrated level and the degree of mixing was clarified.

[0076] First, the definition of the degree of mixing and an example of a
method for determining the degree of mixing will be described. Hereafter,
the determining procedures and the evaluating method will be described in
detail. The present experiment example was performed as follows.

[0077] (1) At 60 seconds after the start of stable stirring, 15 samples
each having a weight of about 100 g are taken from about 8 tons of a coal
blend.

[0078] (2) From each of the samples, one sample having a specified weight
(for example, 1 g) which does not contain particles having a large
particle diameter of more than 6 mm is selected.

[0079] (3) By determining the sulfur concentration of each of the selected
samples, the value is defined as the representative value of the
corresponding sample having a weight of about 100 g. By calculating the
standard deviation of the representative values of the 15 samples having
a weight of about 100 g, the concentrations of which are determined by
the same method, the degree of mixing defined by equation (2) is
calculated.

[0080] Here, .sigma.TS.sub.0 denotes the standard deviation of sulfur
concentrations when mixing is not entirely performed, .sigma.TSf denotes
the standard deviation of sulfur concentrations when mixing has been
completely performed, and .sigma.TS denotes the standard deviation of
sulfur concentrations of the samples taken.

[0081] It is possible to theoretically calculate variations in
characteristic values among samples when mixing has not been performed.
The procedures will be described below. A case is considered where N
samples are taken at random from a coal blend when mixing has not been
performed. At this time, the probability of taking each of the
constituent single coal brands of the coal blend is equivalent to the
blending ratio of the corresponding single coal brand. For example, when
the characteristic value of coal 1 is defined as C.sub.1 and the blending
ratio of the coal is defined as x.sub.1, in the case where ideal random
sampling is realized, the number of samples having a characteristic value
of C.sub.1 is Nx.sub.1. Therefore, the standard deviation of the
characteristic value when mixing has not been performed is calculated by
equation (5) below. In the case where the sulfur concentration in coal is
used as the characteristic value, .sigma.TS.sub.0 described above is
derived.

[0082] Here, .sigma.C.sub.0 denotes the standard deviation of
characteristic values when mixing is not performed at all, i denotes the
identification number of each of constituent single coal brands of a coal
blend, n denotes the total number of constituent single coal brands of a
coal blend, x.sub.1 denotes the blending ratio of a constituent single
coal brand i contained in the coal blend, C denotes the weighted average
value of the characteristic values of the coal blend which is calculated
by equation (6) below, and C.sub.i denotes the characteristic value of a
constituent single coal brand i contained in the coal blend.

C _ = i = 1 n x i C i ( 6 ) ##EQU00003##

[0083] In addition, the standard deviation of characteristic values (for
example, sulfur concentration) when mixing has been completely performed
is estimated as the square root of the unbiased variant which is derived
by performing analysis plural times on well-mixed finely pulverized coal.
This estimation is based on the principle that it is possible to estimate
the standard deviation of a population as the square root of the unbiased
variant among samples taken from the population. Since it is considered
that the standard deviation of characteristic values when mixing has been
completely performed is an analysis error (that is, a standard deviation
which is derived by performing analysis plural times on the completely
same sample), an already-known analysis error may be used. In addition,
as a simplified method, .sigma.Cf (.sigma.TSf in the example described
above) may be assigned a value of 0. Since this simplified method is
mathematically reasonable in the case of an analysis with sufficiently
high accuracy, and since .sigma.Cf takes a constant value, this
simplified method may be used as a simplified method of operation
control.

[0084] In addition, when the degree of mixing is derived, it is preferable
that the degree of mixing be a value calculated from the standard
deviation of a characteristic value which is determined for each of the
samples having a weight of 2 g or less taken from plural positions of a
stirred and mixed coal blend. By taking each sample having a weight of 2
g or less from plural positions, since there is a large difference in
strength between the case where the degree of mixing is 0.85 or more and
the case where the degree of mixing is less than 0.85, it is possible to
realize the effect of the present disclosure to a higher degree.

[0085] The analysis of sulfur concentration was performed as follows by
using carbon-sulfur analyzer EMIA-810 manufactured by HORIBA, Ltd. A
sample weighing 0.1 g was placed on a combustion boat and covered with
0.7 g of alumina powder. By charging the combustion boat into an electric
furnace at a temperature of 1450.degree. C., by burning the coal in an
oxygen gas stream, and by integrating the concentration of sulfur dioxide
generated for 160 seconds, the concentration of sulfur dioxide was
converted to the sulfur concentration in a coal blend. Here, in order to
evaluate the degree of stirring and mixing of a coal blend, an element
mapping method using an electron probe micro analyzer (EPMA) may be used.
An element mapping method using an EPMA is a method in which a mapping
image is derived by detecting the characteristic X-ray of sulfur induced
by an electron beam. Although it is possible to evaluate the state of
dispersion by performing image analysis on the mapping image of sulfur,
there is a disadvantage in that the method requires high techniques
including one for sample preparation and a long time to perform
determination. On the other hand, a method using a carbon-sulfur
analyzer, which requires short time to analyze one sample, and with which
analysis is easily performed by using a small amount of sample, is more
preferable. Also, there is an advantage in that, since the detection
sensitivity of sulfur is very high, the expensive sensitizer described
below is not needed.

[0086] Here, in order to evaluate the degree of mixing, determination may
be performed by adding a material having a characteristic value different
from the average value as a sensitizer. For example, determination may be
performed by adding a material, as a sensitizer, having a characteristic
value 1.5 times or more the weighted average value calculated by equation
(6) above in an amount of more than 0.001 times and of less than 1 times
the total amount of a coal blend. As long as a sensitizer does not have
substantive negative effect on coke strength, coke may be manufactured by
performing carbonization with the sensitizer remaining added. For
example, although sulfur is a chemical element which is originally
contained in coal, a sensitizer having a large sulfur content may be
added in order to perform analysis with a higher sensitivity. It is
particularly preferable to use, as a sensitizer, oil coke, which is
blended as an alternative to coal, or a binder such as coal tar pitch or
asphalt pitch, which is added to a coal blend in order to improve coke
strength in the coke manufacturing process.

[0087] In addition, it is preferable to use a coal blend having a value of
(.sigma.C.sub.0-.sigma.Cf)/Cave of 0.40 or more. In order to increase the
evaluation precision of the degree of mixing, it is preferable that
.sigma.C.sub.0, which is the standard deviation of characteristic values
when mixing has not been performed, be large. From the results of the
investigations regarding various conditions conducted by the present
inventors, it was found that it is preferable to use a coal blend having
a characteristic value with which (.sigma.C.sub.0-.sigma.Cf)/Cave is 0.40
or more, or more preferably 0.55 or more. Here Cave is the average value
of determined characteristic values. As Cave, the average value of the
characteristic values of the constituent coal brands of a coal blend
weighted by the blending ratios of the constituent coal brands may be
used. Also, in the case where .sigma.Cf is assigned a value of 0, a
characteristic value with which .sigma.C.sub.0/Cave is 0.42 or more, or
preferably 0.57 or more, may be used.

[0088] Hereafter, the definition of the disintegrated level and an example
of a method for measuring the disintegrated level will be described.

[0089] (1) Coal to which a powder fluorescent paint (FX-305 manufactured
by SINLOIHI. CO. LTD) is applied is used as a tracer. This fluorescent
paint is characterized by emitting light under ultraviolet irradiation.

[0090] (2) By adding this tracer to a coal blend so that the area fraction
of particles having a particle diameter of 1 [mm] or more is about 5 [%],
by controlling moisture content to be 10 [mass %], a stirring and mixing
operation is performed. (3) The photograph of this coal blend is obtained
by using a digital camera under ultraviolet irradiation. Since the tracer
yields fluorescence in the obtained photograph, by setting appropriate
threshold values of, for example, luminance and brightness in order to
extract only tracer particles, the particle diameter thereof is
determined. Here, the particle diameter of the tracer particle may be
defined as the average value of diameters which connect two points on the
circumference of the extracted tracer particle, which pass through the
center of gravity, and which are determined at intervals of 2 [.degree.].
Also, the particle diameter of the tracer particle may be defined as a
circle-equivalent diameter which is derived by performing image analysis
on a photograph obtained by using a digital camera.

[0091] (4) The disintegrated level is calculated by substituting the
determined particle diameter after a stirring and mixing operation into
equation (7) below.

disintegrated level=1-A/A.sub.0 (7)

[0092] Here, in equation (7), parameter A denotes the area fraction of
particles having a particle diameter of 1 [mm] or more after a stirring
and mixing operation, and A.sub.0 denotes the initial area fraction of
particles having a particle diameter of 1 [mm] or more (about 5 [%]).
That as the disintegrating of pseudo-particles progresses, the
disintegrated level approaches 1.

Experiment 3

[0093] The present inventors evaluated the disintegrated level and the
degree of mixing by using five kinds of mixers of different types in
terms of stirring and mixing method and capability and by performing a
stirring and mixing treatment for a certain time on a coal blend to which
a sensitizer was added and whose moisture content was controlled to be 10
[mass %]. By taking 15 samples each having a weight of 1 g from a coal
blend, and by determining the sulfur concentration of each of the
samples, the degree of mixing was calculated from the determined values.
Among the five kinds of mixers, mixer A was a drum mixer which was widely
used in conventional coke plants and which mainly involves convective
mixing. Mixers C through E were mixers of a shear-mixing type, and mixer
B was a mixer in which convex mixing and shear mixing occurred in
combination. Here, the term "convex mixing" refers to mixing mainly
involving the convection and diffusion of a sample, and the term "shear
mixing" refers to mixing involving shearing, collision, abrasion, and so
forth of a sample.

[0094] FIG. 6 illustrates the relationship between the stirring and mixing
time of each of the mixers and the degree of mixing. In addition, FIG. 7
illustrates the relationship between the degree of mixing after 60
seconds of stirring and the disintegrated level. In FIG. 7, in the
ascending order of the degree of mixing and the disintegrated level, the
points of mixers A, B, C, D, and E are arranged.

[0095] As FIG. 7 indicates, it is clarified that the disintegrated level
largely changes in a range corresponding to a degree of mixing of 0.75 to
0.85. That is, in the case where the degree of mixing of a coal blend is
0.85 or more, or preferably 0.9 or more, pseudo-particles are
disintegrated, and it is possible to manufacture coke having homogeneity
(on the order of millimeters). As described above, in the present
disclosure, by taking samples from any positions of a coal blend before
and after a stirring and mixing step, by determining the characteristic
value of each of the samples, and by calculating the degree of mixing
from the standard deviation of the characteristic value among the samples
by using equation (1) above, the homogeneity of the coal blend is
evaluated in terms of the degree of mixing. For example, by taking
samples from desired positions of a coal blend before and after a
stirring and mixing step, by determining the sulfur concentration of each
of the samples, and by calculating the degree of mixing from the standard
deviation of sulfur concentration among the samples by using equation (2)
above, the homogeneity of the coal blend is evaluated in terms of the
degree of mixing. In addition, when coke is manufactured, stirring and
mixing are performed so that the degree of mixing is 0.85 or more. This
is based on the fact that, as described in the EXAMPLES below, from the
results of the investigations regarding the relationship between the
degree of mixing of a coal blend and the strength of coke which is
manufactured by carbonizing the coal blend, it is possible to obtain coke
having sufficiently high strength in the case where the degree of mixing
of the coal blend is 0.85 or more. Examples of a method for stirring and
mixing include one using a mixing apparatus having a capability of
controlling the degree of mixing of a coal blend to be 0.85 or more at 60
seconds after the start of the mixing operation.

[0096] As FIG. 6 indicates, since the degree of mixing of a coal blend
after 60 seconds is 0.85 or more in the case of mixers C through E, it is
clarified that it is preferable to use mixers C though E, which mainly
involve shear mixing, in order to manufacture coke in the present
disclosure. In contrast, in the case of drum mixer A, which is used in
conventional coke plants, and which mainly involves convex mixing,
pseudo-particles are not substantially disintegrated. In the case of
mixer B, in which convex mixing and shear mixing occur in combination,
although there was an increase in the degree of mixing to about 0.75 in
the case where the stirring and mixing time was more than 60 seconds,
which means that the disintegrating of pseudo-particles progressed
compared with in the case of mixer A, the degree of mixing of a coal
blend after 60 seconds was less than 0.85. Therefore, even in the case of
a mixer of a convex-mixing type or a mixer of a type in which convex
mixing and shear mixing occur in combination, as long as it is possible
to perform stirring and mixing so that the degree of mixing of a coal
blend after 60 seconds is 0.85 or more, or preferably 0.9 or more, such a
mixer may be used for manufacturing coke in the present disclosure,

[0097] Examples of a mixer in a practical operation include one of a batch
type and one of a continuous type in accordance with method of treatment.
A treatment time is equivalent to a stirring and mixing time in the case
of a mixer of a batch type, and an average retention time is equivalent
to a stirring and mixing time in the case of a mixer of a continuous
type. In the case of a mixer of any type, as long as the degree of mixing
is 0.85 or more, or preferably 0.9 or more, when the degree of mixing of
a coal blend is determined after a retention time of 60 seconds, such a
mixer may be used as a preferable apparatus. Since it is necessary to
treat coal in a huge amount of several hundred [t/h] or more for
manufacturing coke, it is preferable that a mixer used in a coke-making
line be a mixer of a continuous type having a high treatment capacity. In
addition, since a coke-making process involves a pulverizing step, a
mixing step, a drying step (including a partially drying step), and so
forth, a coal blend is mixed in a treatment in each of the steps and in
transportation steps, there is a tendency for the coal blend to be
homogenized. Therefore, it is preferable that a stirring and mixing
treatment using a mixer be performed as shortly as possible before the
coal blend is charged into a coke oven if it is performed after a mixing
step from the viewpoint of homogeneity and efficiency.

[0098] Here, it is not necessary to vaporize all the water in coal in a
drying step, and examples of a drying step include a partially drying
step in which moisture content is decreased and a moisture-controlling
step. In addition, a coal blend may contain additives such as binders,
oils, powder coke, oil coke, resins, and wastes.

[0099] [Method for Manufacturing Coke]

[0100] A coal blend is prepared by blending two or more of coal brands.
Subsequently, by stirring and mixing the coal blend which has been
prepared in a preparing step, at least a part of pseudo-particles which
has been formed in the coal blend as a result of coal particles adhering
to each other is disintegrated. At this time, a mixing apparatus having a
capability of controlling the degree of mixing of the coal blend, which
is calculated by equation (1) above, to be 0.85 or more at 60 seconds
after the start of the mixing operation is used. Moreover, the coal blend
which has been subjected to a stirring and mixing step is charged into a
coke oven and carbonized. As described above, coke is manufactured.

[0101] Here, when a coal blend is prepared, it is preferable that the two
or more coal brands be pulverized before the two or more coal brands are
blended. By thus pulverizing the two or more of coal brands before the
two or more coal brands are blended, there is an increase in the effect
of increasing coke strength as a result of stirring and mixing.

[0102] In addition, when a stirring and mixing treatment is performed, it
is preferable that a stirring and mixing treatment is performed on a coal
blend having a moisture content of 6 mass % or more from the viewpoint of
clearance. In addition, in the case where the moisture content of a coal
blend is more than 6 mass % when a stirring and mixing treatment is
performed, there is an increase in the effect of increasing coke strength
as a result of performing a stirring and mixing treatment so that the
degree of mixing is 0.85 or more compared with coke strength in the case
where a stirring and mixing treatment is not performed or where a
stirring and mixing treatment is insufficiently performed. Therefore, it
is more preferable that stirring and mixing be performed on a coal blend
having a moisture content of more than 6 mass %.

Example 1

[0103] By adding water to four kinds of single coal brands (coal A through
coal D) having the properties given in Table 1, and by leaving the single
coal brands untouched for 24 hours in order to obtain homogeneous
moisture content, the moisture content was controlled to be 3 [mass %] to
14 [mass %]. By using the mixers A through E described above involving
different types of stirring and mixing functions, these single coal
brands were subjected to stirring and mixing for 60 seconds in order to
prepare coal blends having blending ratios given in Table 1. By filling a
carbonizing vessel with 17.1 [kg] of each of the prepared coal blends so
that the bulk density (based on dry weight) was 725 [kg/m.sup.3], by
carbonizing the coal blend with a weight of 10 [kg] being placed on the
top of the carbonizing vessel in an electric furnace having a furnace
wall temperature of 1050 [.degree. C.] for 6 hours, by removing the
carbonizing vessel from the furnace, and then by cooling the carbonizing
vessel with nitrogen gas, coke was obtained. The drum index DI (150/15)
and clearance of the obtained coke were determined. The method for
determining drum index DI (150/15) was as described above. Clearance was
determined as follows.

[0104] By filling a small simulation retort for determining clearance with
2.244 [kg] of the coal blend having a bulk density (based on dry weight)
of 775 [kg/m.sup.3], the coal blend was carbonized in an electric furnace
having a furnace wall temperature of 1050 [.degree. C.] for 4 hours and
20 minutes. The retort was removed from the furnace and cooled with
nitrogen gas. A gap between the side surface of the obtained coke cake
and each of the side panels on the right and left sides was determined by
using a laser distance meter. By calculating the average value of the gap
on each of the sides, the sum of the gaps on both sides was defined as a
clearance.

[0105] The moisture content when mixing was performed, drum index DI
(150/15), and clearance of each of the samples are given in Table 5.

[0106] As Table 5 indicates, it is clarified that, by performing stirring
and mixing by using any one of the mixers C, D, and E having a capability
of controlling the degree of mixing of a coal blend after 60 seconds of
stirring and mixing to be 0.85 or more, or preferably 0.9 or more, since
the disintegrating of pseudo-particles progressed, it was possible to
manufacture coke excellent in terms of both coke strength and clearance
even in the case where moisture content was 6 [mass %] or more. That is,
in the case where mixer A or B was used, coke strength achieved was much
smaller in the case where moisture content was more than 6 mass % than in
the case where moisture content was 6 mass %. On the contrary, by using
mixer C, D, or E with which the degree of mixing of a coal blend after 60
seconds of stirring and mixing was 0.85 or more, coke strength which was
achieved in the case where moisture content was more than 6 mass % is
almost equal to that in the case where moisture content was 3 mass % to 6
mass %, which means that the effect of increasing coke strength as a
result of stirring was large. Here, although no difference in determined
clearance resulting from variations in the moisture content is observed
in Table 5 because carbonization was performed with a constant bulk
density in the testing method described above, it is known that, in a
practical operation, in the case where moisture content is high, since
there is a decrease in the bulk density of coal charged into the
carbonization chamber of a coke oven on a dry basis, there is an increase
in the amount of shrinking. Therefore, it is particularly preferable that
the method according to the present disclosure be used in the case where
a coal blend having a high moisture content is carbonized.

Example 2

[0107] The present inventors, by evaluating the degree of mixing under
various material conditions, investigated the relationship between the
determined degree of mixing and coke strength.

[0108] Sample size influences the detection sensitivity of the degree of
mixing. That is, the smaller the sample size, the larger the influence of
particles having a characteristic value different from the average value
of a coal blend. In contrast, in the case where a sample size is large,
since particles having various characteristic values are contained in the
sample, there is a decrease in a variation due to averaged characteristic
value. Therefore, the smaller the sample size, the higher the detection
sensitivity of the degree of mixing. On the other hand, it is necessary
to use a certain amount of sample in order to analyze a characteristic
value, and there is an increase in analysis error in the case of a small
sample size. The present inventors conducted investigations regarding how
sample size influences the degree of mixing and the detection sensitivity
of coke strength by performing a carbonization test.

[0109] By using a coal blend (base coal blend) which is practically used
in a commercial coke oven and coal blends which were prepared by adding
delayed oil coke to the base coal blend as a sensitizer in an amount of
0.1% to 50%, the coal blends were subjected to pulverization, blending,
and moisture control. Subsequently, the coal blends having a weight of
300 kg were made into coal blends having various degrees of mixing by
using mixers having various stirring capabilities and by performing
stirring for various periods of time. The properties (mean maximum
reflectance Ro [%], Gieseler maximum fluidity log MF [log ddpm], volatile
matter content VM [mass %], ash matter content Ash [mass %], and total
sulfur content (TS) [mass %]) of single coal brands (coal E through coal
T) and delayed oil coke which were contained in the coal blends used in
the test are given in Table 6, and the average properties of the base
coal blend are given in table 7.

[0110] Coke strength was evaluated through the following procedures. A
laboratory furnace having a capacity of 1/4 tons was used to carbonize
each of the coal blends. By charging about 200 [kg] of a coal blend into
the furnace through free fall, by carbonizing the coal blend in an
electric furnace having a furnace wall temperature of 950 [.degree. C.]
for 23 hours, by then removing the sample from the furnace, and by
cooling the sample with nitrogen gas, coke was obtained. Regarding the
strength of the obtained coke, in accordance with the drum strength test
method prescribed in JIS K 2151, by determining the mass fraction of coke
having a particle diameter of 15 [mm] or more after the coke had been
rotated 150 times at a rotating speed of 15 [rpm], and by calculating the
ratio of the mass fraction to that before the rotation, the ratio
multiplied by 100 was defined as drum index DI (150/15).

[0111] Regarding the degree of mixing of the coal blend, by taking 15
samples having a specified sample size, by determining total sulfur
content in accordance with JIS M 8813, the degree of mixing was
calculated by equation (1) above. Here, .sigma.C.sub.0 of coal blend was
0.18 mass % for the base coal blend and 0.20 mass % to 1.31 mass % for
the coal blends containing delayed oil coke, and
(.sigma.C.sub.0-.sigma.Cf)/Cave was 0.33 for the base coal blend and 0.36
to 1.00 for the coal blends containing delayed oil coke. In addition,
.sigma.Cf was 0.008 mass % in any case.

[0112] FIG. 8 illustrates the relationship between the blending ratio of
delayed oil coke and (.sigma.C.sub.0-.sigma.Cf)/Cave. As FIG. 8
indicates, it is clarified that (.sigma.C.sub.0-.sigma.Cf)/Cave has a
maximal value when plotted against the blending ratio of a sensitizer.

[0113] Subsequently, by using a coal blend having a value of
(.sigma.C.sub.0-.sigma.Cf)/Cave of 1.00 which was obtained by adding
delayed oil coke to the base coal blend, and by determining the degrees
of mixing for various sample sizes, the relationship between the
determined degree of mixing and the strength of coke which was
manufactured by carbonizing the coal blend was investigated. The results
are illustrated in FIG. 9. The points indicating the same strength
represent the coke which was manufactured from the same coal blend. As
FIG. 9 indicates, it is clarified that, since the determined degree of
mixing of the same coal blend varies depending on sample size, plural
points are plotted for the same value of strength. It is clarified that,
the larger the sample size, the smaller the difference between the
maximum and minimum values of the determined degree of mixing, which
results in a decrease in the detection sensitivity of the degree of
mixing. It is clarified that, in the case where the sample size was 15 g
or less, there is a tendency for coke strength to improve with improving
degree of mixing. Therefore, it is clarified that, in order to detect the
degree of mixing, it is preferable that the sample size be 15 g or less,
or more preferably 2 g or less. It is possible to determine the lower
limit of the sample size from the viewpoint of a method for analyzing a
characteristic value, and it is preferable that the lower limit be 0.1 g
or more.

[0114] As FIG. 9 indicates, it is clarified that, in the case where the
sample size was 2 g or less, there was a large difference in coke
strength between the case where the degree of mixing was 0.85 or more and
the case where the degree of mixing was less than 0.85. It is clarified
that it is preferable that stirring be performed so that the degree of
mixing is 0.85 or more in order to maintain a high level of coke
strength.

Example 3

[0115] The influence of (.sigma.C.sub.0-.sigma.Cf)/Cave on the determined
value of the degree of mixing was investigated. By adding delayed oil
coke to the base coal blend in various amounts, coal blends having
various values of (.sigma.C.sub.0-.sigma.Cf)/Cave were prepared.
Subsequently, by stirring the coal blends by using mixers having various
stirring capability, the degree of mixing after stirring was determined
with a sample size of 1 g. In Table 8, the degrees of mixing determined
after stirring by using mixer B having the highest stirring capability
and mixer A having the lowest stirring capability and the difference in
the degree of mixing between the two mixers are given.

[0116] As Table 8 indicates, it is clarified that, in the case where
(.sigma.C.sub.0-.sigma.Cf)/Cave was 0.36 or less, there was almost no
difference in the degree of mixing determined after a mixing step between
mixers A and E. On the other hand, in the case where
(.sigma.C.sub.0-.sigma.Cf)/Cave was 0.40 or more, since there was an
increase in a difference in the degree of mixing, it was possible to
detect a difference in the degree of mixing. From the results described
above, it is clarified that it is preferable to use a coal blend having a
value of (.sigma.C.sub.0-.sigma.Cf)/Cave of 0.40 or more, or more
preferably 0.55 or more. Here, at this time, in the case where
(.sigma.C.sub.0-.sigma.Cf)/Cave was 0.40 or more, coke strength was 82.5
or more for a degree of mixing of 0.85 or more, and coke strength was
less than 82.5 for a degree of mixing of less than 0.85.

Example 4

[0117] By evaluating the degree of mixing of a mixer used in a practical
coke oven with the method according to the present disclosure, coke
strength was evaluated. In the final stage of a pretreatment process in a
coke-making line, that is immediately before a transportation step
through which coal was carried to a coke oven, the mixer was installed.
The mixer was of a continuous type and had a capability of controlling
the degree of mixing to be 0.85 or more after 60 seconds of a mixing
operation (that is, after an average retention time of 60 seconds).

[0118] The change in the degree of mixing of a coal blend according to a
treatment time in the mixer was investigated. Total sulfur content
prescribed in JIS M 8813 was used as a characteristic value. Here,
delayed oil coke was added to the coal blend in an amount of 10%. At that
time, .sigma.TS.sub.0 was 0.98, and (.sigma.TS.sub.0-.sigma.TSf)/TSave
was 0.99. Each of the populations of the coal blend taken on the belt
conveyers on the inlet and exit sides of the mixer had a weight of about
6 tons. By taking plural specimens having a weight of about 1.2 kg from
the population by using a sampling shovel in accordance with JIS M
8811-50, and 15 samples having a sample size of about 1 g was taken from
each of the specimens. Total sulfur content of each of the samples was
determined in accordance with JIS M 8813. The degree of mixing was
calculated by equation (2) above. FIG. 10 illustrates the obtained
results. As FIG. 10 indicates, it is clarified that, the longer the
average retention time in the mixer, the higher the degree of mixing.

[0119] A change in the strength of coke manufactured by using a practical
coke oven due to the installed mixer was investigated. FIG. 11
illustrates coke strengths in the case where a treatment using the mixer
was not performed (the degree of mixing.apprxeq.0.74) and in the case
where a treatment using the mixer was performed for an average retention
time of about 60 seconds (the degree of mixing 0.90%). Here, coke
strength was determined every 8 hours during the test period. As FIG. 11
indicates, from the results of t-test with a confidence interval of 95%
on both sides, it is clarified that there was an improvement in drum
strength due to a stirring treatment using the mixer with a significant
difference. In addition, from the results of F-test with a confidence
interval of 95% on both sides, it is clarified that there was a decrease
in a variation in strength with a significant difference. The reason why
there was a decrease in a variation in strength is considered to be
because, as a result of installing the mixer, there was an improvement
not only in homogeneity on the order of millimeters but also in macro
homogeneity.

[0120] As described above, by using the degree of mixing evaluated in the
present disclosure as an index, and by performing an operation in order
to improve the degree of mixing, it is possible to achieve an improvement
in coke strength and a decrease in a variation in coke strength.

[0121] Although the exemplary embodiments of the present disclosure by the
present inventors have been described above, the present disclosure
includes, but is not limited to, the descriptions and figures of the
present embodiments. That is, other embodiments, working examples, and
operational techniques and the like, which are performed on the basis of
the present embodiments by those with an ordinary skill in the art are
all within the scope of the present disclosure.